Tl{hd 3 {b PSe{hd 4 {b compound, single crystals

ABSTRACT

The compound Tl3PSe4 is disclosed. Non-conducting single crystals of the compound are prepared which have outstanding acousto-optical properties including an exceptionally high acousto-optical figure of merit. The crystals are used in various acousto-optical devices including a display device, a laser modulator, a non-collinear acousto-optic filter, and an acoustic delay line.

United States Patent [1 1 Isaacs et al.

[ Dec. 30, 1975 [54] TL; ISE' COMPOUND, SINGLE CRYSTALS [75] Inventors:Thelma J. Isaacs, Monroeville;

Milton S. Gottlieb, Pittsburgh; John D. Feichtner, Murrysville; AndreaA. Price, Pittsburgh, all of Pa.

Westinghouse Electric Corporation, Pittsburgh, Pa.

22 Filed: Aug. 29, 1973 21 Appl. No.: 392,693

[73] Assignee:

[52] US. Cl. 423/299; 423/508; 350/161; 333/30 R [51] Int. Cl. C01B25/00 [58] Field of Search 423/299, 304-306, 423/508 [56] ReferencesCited OTHER PUBLICATIONS Acta Crystallallographica, C. Crevecoeur, No.17,

1964, p. 757 ff.

Chemical Abstracts, Vol. 68, 1968 457l3e.

Chemical Abstracts, Vol. 70, 1969 621125.

Chemical Abstracts, Vol. 72, 1970 94498e.

Primary ExaminerOscar R. Vertiz Assistant ExaminerGregory A. HellerAttorney, Agent, or FirmR. D. Fuerle [57] ABSTRACT 7 Claims, 5 DrawingFigures US. Patent Dec. 30, 1975 GENERATOR vGENERATOR GENERATOR RFGENERATOR TL PSE COMPOUND, SINGLE CRYSTALS CROSS-REFERENCE TO RELATEDAPPLICATIONS This application is related to application Ser. No. 242,986filed Apr. 11, 1972, entitled Tl AsS Crystals And Acousto-OpticalSystem, now U.S. Pat. No. 3,799,659.

This application is related to application Ser. No.

392,695, filed Aug. 29, 1973 by T. J. Isaacs et al. titled TI VS and TlNbS Crystals and Acousto-Optical Devices.

This application is related to application Ser. No. 540,192 filed Jan.10, 1975 by T. J. Isaacs et al. titled Tl TaS and Tl TaSe Crystals andAcousto-Optical Devices.

BACKGROUND OF THE INVENTION In 1932 Brillouin discovered that highfrequency sound waves can cause diffraction of light. With the advent ofthe laser and advances in high frequency acoustic techniques manyapplications for this phenomenon have been found such as display devicesor laser modulators.

A sound wave moving in crystal is composed of alternating compressionand rarefaction fronts. The indices of refraction in these fronts aredifferent, so that the crystal acts as a diffraction grating,diffracting light which passes through it, the angle of diffractionincreasing as the frequency of the sound wave increases, and the amountof light diffracted increasing with the intensity of the sound wave.

There are two modes of diffraction, the Debye-Sears mode and the Braggmode. The Debye-Sears mode is obtained if the width of the acoustic beamis less than about A /(4A) and the Bragg mode is obtained if the widthof the acoustic beam is greater than about A /(4A) where A is theacoustic wavelength and A is the light wavelength. In both modes theacoustic wavelength A must be greater than the light wavelength A, and)1 must be within the transparency region of the crystal. In theDebye-Sears mode light enters the crystal parallel to the acoustic wavefronts diffracting angle) and is multiply-diffracted into many images ororders of the initial light beam. In the Bragg mode light enters thecrystal at the Bragg angle (I) to the acoustic wave fronts where sin (b)t/A. In this mode the acoustic wavelength and the Bragg angle arematched to the particular light wavelength, and a single image isdiffracted from the crystal at the Bragg angle (1) to the acoustic wavefronts.

A good acousto-optical material should have a high figure of merit, M ameasurement of the amount of light diffracted for a given amount ofacoustic power, where M m p /p v and n is a refractive index, p is thephotoelastic coefficient, p is the density, and v is the acousticvelocity. As the formula indicates, a low velocity will give a highfigure of merit and, in addition, it will give a greater delay per unitlength if the crystal is used in a delay line thus permitting acousticsignal processing devices to have smaller physical dimensions. A goodacousto-optical material should also have a low acoustic attenuation,allowing a high frequency wave to propagate a-long distance before it isabsorbed.

The following table gives a few of the properties of the bestacousto-optical materials currently known for use in the near infraredregion of the spectrum.

Optical Transmission Acoustic Velocity Fig.

Material Range (m) (X 10 cm/sec) of Merit Ge 2-20 5.5 525 AS253 glass0.9-1 1 2.6 230 aAs l-l 1 5.15 93 Tl Ass, 0.6-12 2.15 330 PbMoO, 0.4-5.53.83 24 PRIOR ART An article entitled Some Ternary ThalliumChalcogenides by C. Crevecoeur appears in the January-- June 1964 volume(No. 17) of Acta Crystallographica on page 757. That article describesthe preparation and characteristics of the isomophous compounds TI VS TlNbS Tl TaS Tl VSe Tl NbSe and Tl TaSe SUMMARY OF THE INVENTION We havediscovered the existence of the compound Tl PSe and have found thatlarge single crystals can be grown from this compound. We have alsofound that crystals of Tl PSe have the highest acousto-optical figure ofmerit and lowest acoustic velocity of any known material, and thatacousto-optical devices employing these crystals have outstandingproperties. The

crystals also have good physical properties such as resistance tomoisture, they can be grown to large sizes, and they have low acousticvelocities and acoustic attenuation.

DESCRIPTION OF THE INVENTION FIG. 1 is an isometric diagrammatic drawingof a display device;

FIG. 2 is a diagrammatic drawing of a laser modulator of the internalconfiguration;

FIG. 3 is a diagrammatic drawing of a laser modulator of the externalconfiguration;

FIG. 4 is a diagrammatic drawing of an acoustic delay line;

FIG. 5 is a diagrammatic drawing of a non-collinear acousto-opticalfilter.

PREPARATION OF THE COMPOUND AND CRYSTAL The compound of this invention,T1 PSe may be prepared by mixing together very pure stoichiometricquantities of the elements involved and melting them together until theyhave reacted to form the compound. The compound and the resultingcrystal may be made slightly non-stoichiometric in order to relieveinternal stresses. Up to about 50% of the phosphorus may be substitutedfor with arsenic.

The crystal may be prepared by the Stockbarger technique in which thecompound is sealed in a quartz tube under argon, melted, and loweredvery slowly (10 to 15 mm/day) through a two-zone furnace having a steeptemperature gradient (8 to 12C/mm) at the melting point of the compound.The compound melts congruently at approximately 436C i 10C.

THE CRYSTAL The crystal of this invention is biaxial nonpiezoelectrio,and orthorhombic. Its space group is Pcmm, its Laue class is mmm, andthe diffraction aspect derived from X-ray data is Pc*n. The length ofthe axes of the 3 crystal are about a= 9.270A, b= 11.047A, c= 9.059A,and its transparency region is about 0.78 n m to about l7 p. m, althoughstrong absorption peaks exist at 9.53 p. m, 9.76 p. m, from 11.4 p, m to12.5 pt m, and from l5.6p.mto l6.lp.m. i

The crystals should be as long as possible in order to maximize theoutput power, but if the crystal is too thick (i.e., more than about 10cm) light loss due to absorption will be high. On the other hand, thecrystal should not be too thin in the direction of light propagation asthis will result in poor interaction between the light and sound andtherefore a low intensity defraction, but a crystal as small as 1 mmlong can be optically useful. From the practical point of view oforienting and polishing faces on the crystal and attaching to it atransducer to generate acoustic waves, the crystal must have dimensionsof at least .1 mm. The width of the crystal should be at least as wideas the input beam can be focused, about lO mm, so that the light is notwasted. For acousto-optical applications, the crystal must be largeenough to produce a Bragg interaction between sound and light. Thatrequires at least 10 acoustic wave fronts, which means a minimum lengthabout 2 X l mm is required at an acoustic frequency of 300 MHz.Preferably the crystal should be at least about .05 cm in diameter andabout 1 cm long to have practical usefulness in most applications. Thecrystal also preferably has at least two polished parallel opticalfaces, which preferably are perpendicular to those axes of the crystalalong which sound propagates as a pure longitudinal or skew mode.

THE SOUND WAVES The sound wave may be a longitudinal wave, where theparticle motion is in the direction of propagation of the wave, or itmaay be a shear wave, where the particle motion is perpendicular to thepropagation direction of the wave, or it may be a combination of both.Preferably, it is either pure shear or pure longitudinal because the twowaves travel at different velocities and quickly become out of phase.For delay line applications shear waves are desirable because of theirlower velocity. Pure shear waves are obtained by propagating the wave ina pure shear direction (determined from the crystal symmetry) using ashear wave generating transducer such as a Y cut or A-C cut quartz,which is glued to the crystal. Longitudinal waves are obtained bypropagating the wave along the c-axis or another pure longitudinaldirection using a longitudinal wave generating transducer such as X-cutquartz which is glued to the crystal.

DISPLAY DEVICES In a display device a light beam is directed at thecrystal and the deflected beam which leaves the crystal is directed atsome type of viewing screen.

In FIG. 1 RF generators 1 and 2 send RF signals to transducers 3 and 4respectively which respectively generate vertically moving andhorizontally moving sound waves in crystal 5, preferably in the Braggmode so that there is only one diffracted beam. The light, which ispreferably parallel and polarized for good resolution, is obtained fromlaser 6 which generates a coherent beam of light 7 directed at one ofthe two parallel optical faces 8 of crystal 5. Light passing throughcrystal is directed at various spots 9 on viewing screen 10 by means ofthe vertically and horizon- 4 tally moving sound waves generated bytransducers 3 and 4. Lens 11 focuses the light at the spot.

The illuminated spots may each be a page of information which is thenoptically enlarged and projected on a second viewing screen (not shown).The illuminated spots could also in themselves form a pattern. Forexample, viewing screen 10 could be an infrared-sensitive phosphorcoated screen such as zinc sulfide doped with lead and copper andflooded with UV light and the successive illumination of selected spotswould form a picture similar to a TV picture. Or, viewing screen 10could be infrared or thermally quenched UV-excited phosphor screen whereultraviolet light causes the entire screen to be illuminated, but eachselected spot successively struck by the beam from crystal 5 is darkenedto form a picture on the screen.

LASER MODULATOR In a laser modulator the acousto-optical systemmodulates a portion of the output of the lasing medium. If the light isfocused to less than about 10 or l0 cm it will be modulated but notdiffracted. For greater diameter focal spots it will be both diffractedand modulated. A laser modulator could be used, for example, to sendsignals by means of the fluctuating laser beam intensity.

FIG. 2 shows a laser modulator of the internal configuration. In FIG. 2,lasing medium 12 produces a beam of coherent light which ismultiply-relfected between mirrors l3 and 14. Mirror 13 totally reflectsthe light and mirror 14 partially reflects it and partially transmits itas the laser output 32. interposed between lasing medium 12 and mirror14 is a crystal 15 of Tl PSe (The crystal could also be positionedbetween mirror 13 and the lasing medium). To crystal 15 is afflxed atransducer 16 which is electrically connected to an RF generator 17.This generator produces a radio-frequency electrical signal whichtransducer 16 coverts into an acoustic wave which moves through crystal15 diffracting light as shown at 18.

FIG. 3 shows a laser modulator of the external configuration. In FIG. 3lasing medium 19 produces a beam of coherent light which ismultiply-reflected between mirror 20, which totally reflects the beam,and mirror 21 which partially reflects the beam and partially transmitsit as laser output 22. The laser output 22 strikes crystal 23 of Tl PSeto which is affixed transducer 24 electrically connected to RF generator25. Generating a sound wave in the crystal diffracts the laser outputcausing it to strike screen 26 instead of passing through aperture 27 inthe screen.

ACOUSTIC DELAY LINE An acoustic delay line causes an electrical signalto be delayed for the length of time required for an acoustic signal totraverse the crystal, L/ V, where L is the length of the crystal and Vis the acoustic velocity. Unlike many other methods of delaying anelectrical signal, an acoustic delay line preserves the originalconfiguration of the signal.

In FIG. 4, RF generator 28 provides the electrical signal to be delayed.This signal is electrically transmitted to transducer 29 which convertsthe signal to an acoustic wave which is propagated through crystal 30 ofTl PSe At the other end of the crystal transducer 31 detects theacoustic wave and converts it into an electrical signal.

NON-COLLINEAR FILTER In a non-collinear filter, the incident light 32strikes the crystal 33 at a fixed angle, 4). Only light of wavelength A,which satisfies the condition U -sinqS will be diffracted at the angle(b into the output beams 34; f is the frequency applied to thetransducer 35. Light of any other wavelength passes through the crystalundeflected. Any wavelength of light may be selected for deflection bychoosing the appropriate frequency.

EXAMPLE A reaction vessel was charged with 6.1311 grams thallium, 0.3097grams phosphorus, and 3.1584 grams selenium. The vessel was sealed undervacuum and heated at about 800C for 1 day. It was shaken vigorously anumber a times for thorough mixing in order to produce the compound TlPSe The reactant was placed in a fused quartz crystal growing tube 0.8centimeters in diameter and covered with argon at a pressure of 15inches. Using the Stockbarger technique, crystal Tl PSe was grown fromthe melt at a rate of 13.5 millimeters/day. The crystal was cutperpendicular to the axes to form a cube about 0.6 cm. on a side.

The refractive indices of the crystal were measured in a method ofnormal incidence, using a spectrometer with a chopped tungsten source.Wavelength selection was accomplished with narrow band interferencefilters, and detection of the deflected beam was by a photo-multiplierfor the 0.8 p. m to 1.15 y. m region, and by liquid nitrogen cooled lnSbphoto-voltail detector for wavelength of the region 1.2 u m to 5.3 p. m.The following table gives the results of the refractive indexmeasurements.

Refractive Indices Refractive Indices-continued (n,, is refractive indexfor light polarized l to the a-axis, where a, b, and c are thecrystallographically defined axes such that a 9.270 A, b 11.047 A, c9.059 A).

The acoustic properties of the crystal were measured on a sample ofcrystal approximately 0.5 centimeters on the side. Transducers werecemented on each part of opposite faces of the crystal, and thevelocities were measured by the conventional pulse-echo method. Thelongitudinal wave velocity for propagation along the c-axis was 2.22 X10 cm/sec, and along the aand b-axes it was 1.98 X 10 cm/sec. There wasa fast shear wave for propagation along each of the three axes, ofvelocity 1.1 X 10 cm/sec. For propagation along the c-direction and thea-direction there is also a slow shear wave of velocity 5.05 X10 cm/sec.

The acousto-optic figure of merit at )t 1.15pm, relative to fused quartzwas measured for various configurations. For longitudinal waves, thisrelative figure of merit ranged from 500 to 1365, and for shear waves,it reached a measured value of 1370.

We claim:

1. A compound having the general formula Tl XSe where X is about 50 toabout phosphorus and about 0 to about 50% arsenic.

2. A single crystal of the compound of claim 1.

3. A compound according to claim 1 having the formula Tl PSe 4. Abiaxial, non-piezoelectric, orthorhombic single crystal according toclaim 2 having the formula 11313864.

5. A single crystal according to claim 2 which is at least about 10 cmwide and 1 mm long.

6. A single crystal according to claim 5 which is at least about 0.5 cmin diameter and at least about 1 cm long.

7. A single crystal according to claim 5 having two parallel opticalfaces.

1. A COMPOUND HAVING THE GENERAL FORMULA TI3XSE4 WHERE X IS ABOUT 50 TOABOUT 100% PHOSPHORUS AND ABOUT 0 TO ABOUT 50% ARSENIC.
 2. A singlecrystal of the compound of claim
 1. 3. A compound according to claim 1having the formula Tl3PSe4.
 4. A biaxial, non-piezoelectric,orthorhombic single crystal according to claim 2 having the formulaTl3PSe4.
 5. A single crystal according to claim 2 which is at leastabout 10 3 cm wide and 1 mm long.
 6. A single crystal according to claim5 which is at least about 0.5 cm in diameter and at least about 1 cmlong.
 7. A single crystal according to claim 5 having two paralleloptical faces.